Device and method for detecting and locating targets in bistatic mode

Abstract

The invention relates to a method and a device for detecting and locating (RE) targets (C) configured to exploit in bistatic mode non-cooperative radio signals emitted on separate channels by a transmitting source (EM), the device comprising a plurality N of at least two receiving elements (EREi), i ∈ [1 ; N], as well as a processing unit (ECU) of said radio signals, characterized in that: The receiving elements (EREi) are substantially aligned along an axis (x) and, The value of the spacing d(EREi, EREi+1) between two successive receiving elements (EREi, EREi+1) is greater than half the wavelength of said radio signals, and, ∀ i ∈ [1 ; N] and ∀ k ∈ [1 ; N], the value of the spacing d(EREi, EREk) between two distinct receiving elements (EREi, EREk) is unique. Figure 1

Description

Title of the invention: Device and method for detecting and locating targets in bistatic mode technical field

[0001] The present invention relates to passive methods and systems exploiting opportunistic radio and television signals (FM, Digital Audio Broadcasting, Digital Video Broadcasting) to detect and locate targets. These non-cooperative emissions are, for example, broadcast signals such as FM radio or television signals such as DVB (Digital Video Broadcasting). The invention relates in particular to passive bistatic radars operating in the UHF or VHF bands, dedicated to the detection and localization, particularly Cartesian localization, of small aerial targets flying primarily at low altitudes.

[0002] These passive bi-static radars make it possible in particular to detect intrusions even in the absence of a high density of transmitters, on sensitive sites such as military infrastructure, strategic industrial sites.

[0003] These passive bi-static radars also allow for initial classification based on the specific signatures of intruders and / or for characterizing targets of no interest. State of the art

[0004] Passive systems are traditionally viewed as discrete means of detection and localization, primarily for supplementary coverage such as low-altitude detection. However, today, all passive systems that exploit opportunistic transmissions such as television or radio broadcasts with a carrier frequency below 1 GHz face challenges with poor angular measurement. This limitation is all the more significant when they seek to protect infrastructure using omnidirectional receivers employing a few antennas, typically on the order of 8 to 16.

[0005] Such an approach therefore leads to individual components enabling target detection, the estimated angular parameter of which is obtained with poor precision, necessitating triangulation between three bistatic pairs to initiate target tracking. Consequently, while tracking can then be continued and maintained using only two bistatic pairs or even a single one, it can only be initiated within the area of ​​common detections, thus limiting the localization range.

[0006] The present invention is based on an analysis of the limitations of previous approaches and the specific needs associated with various low-altitude protection objectives. This analysis takes into account the points detailed below.

[0007] First of all, it is limiting to consider the antenna array only as a means of restricting direct path blindness and clutter while providing a poor indication of the direction of arrival of targets.

[0008] Furthermore, most sites requiring protection insist on having the protection equipment within their perimeter for obvious reasons of protecting this equipment. Given the desired protection range, the required precision in locating the threat across all sectors, and the possibility of ideally placing receivers within the site to ensure good triangulation, it can be complex to locate threats using the "multistatic association" approach.

[0009] Moreover, some infrastructures, such as power plants, are incompatible with omnidirectional sensors.

[0010] Moreover, for certain situations and scenarios, the density of emitters may be insufficient to consider a simple receiver exploiting three emitters according to the principle of association.

[0011] Furthermore, it should be noted that classical multistatic association localization only allows tracking to be initiated in the common detection zone.

[0012] Based on these observations, the inventors decided to define a method and a device for sectoral surveillance using a single bistatic pair, namely a transmitter and a receiver, allowing direct localization of all targets. Description of the invention

[0013] More specifically, the invention relates to a target detection and localization device, configured to exploit in bistatic mode non-cooperative radio signals emitted on distinct frequency channels by a transmitting source, the device comprising a plurality N of at least two, preferably at least three, receiving elements, such as antennas, as well as a unit for processing said radio signals, characterized in that: - The receiving elements are substantially aligned along an axis and, - The value of the spacing between two successive receiving elements is greater than half the wavelength of said radio signals, and, - The value of the spacing between two distinct receiving elements is unique.

[0014] The fact that this linear device is lacunar, that is to say that the spacing between two successive receiving elements is greater than half the wavelength of the radio signals, makes it possible to obtain an improved resolution.

[0015] The fact that this linear device is irregular, that is to say that the spacings between any two receiving elements are all different from each other, makes it possible to limit ambiguities.

[0016] Various embodiments of the invention are provided, incorporating, according to all their possible combinations, the different optional features set out here.

[0017] Preferably, the minimum difference between the values ​​of the spacings is at least equal to 10%.

[0018] Thus, the spacings between any two receiving elements are clearly differentiable, which makes it possible to eliminate ambiguities.

[0019] Advantageously, the value of the spacing between two successive receiving elements is shaped to exploit radio signals used for broadcasting analog and digital television or radio, after adaptation to transpose the dimensioning carried out in the television frequency band to the radio frequency band.

[0020] This allows easy access to sources of opportunity throughout the territory.

[0021] Advantageously, the value of the spacing between the first receiving element and the last receiving element is less than 20 m.

[0022] The device according to the invention can thus be easily installed on a roof or a building terrace.

[0023] Preferably, the target detection and localization device comprises at least 4 receiving elements.

[0024] Preferably, the target detection and localization device comprises at most 10 receiving elements.

[0025] Preferably, the target detection and localization device comprises between 7 and 9 receiving elements. This allows for target detection and localization with minimized equipment costs.

[0026] The invention also relates to a target detection and localization system comprising a plurality of target detection and localization devices according to the invention and arranged to cover an angular sector of 360°.

[0027] The system thus constituted makes it possible to ensure omnidirectional surveillance.

[0028] The invention also relates to a method for detecting and locating targets using a target detection and locating device that exploits non-cooperative radio signals emitted on separate channels by a source emitting, said device conforming to an embodiment of the invention, said method comprising at least: - A reception stage by each receiving element of the target detection and localization device, of a signal originating from the emitting source along a direct path, and of a signal backscattered by a target in the covered area, - A selection step enabling, for each receiving element of the target detection and localization device, the separation and selection of at least one frequency channel to be used, - A step of determining the bistatic distance (d) and the radial velocity (vr) of the target, itself derived from said bistatic distance, - A step of determining the angle of arrival 0O of the target, from the measurements of the phase differences between the signals received by each receiving element of the target detection and localization device.

[0029] Various embodiments of the invention are provided, incorporating, according to all their possible combinations, the different optional features set out here.

[0030] According to a preferred embodiment, the step of determining the bistatic distance (d) and the radial velocity (vr) of the target is implemented by means of a distance-Doppler correlation operation on the received signal, with replicas of the signal emitted by the emitting source, affected by time and frequency shifts.

[0031] In particular, the step of determining the bistatic distance (d) and the radial velocity (vr) of the target implements the following sub-steps: - The expression of the signal received by a receiving element (ERE;) as a function of the signal received by a preselected reference receiving element (EREref): « / , \ , rd <EREi.EREnfKoi&,) \

[0032] With

[0033] srî; signal received by the receiving element (ERE;),

[0034] se; signal emitted by the emitting source (EM),

[0035] fo: frequency of the emitting source (EM),

[0036] T“: bistatic delay (common to all receiving elements (ERE;)) between the path from the emitting source (EM) to the target, and the path from the target to the receiving element (ERE);

[0037] : derivative of the delay T° dt

[0038] _ : Bistatic Doppler vo~ J u dt

[0039] d ( EREi, EREref ) : value of the spacing between the receiving element (ERE;) and the predetermined reference receiving element (EREref)

[0040] : angle of arrival of the target common to all receiving elements (ERE;) - Estimation of parameters and % using the search for the maximum power of the function v): v) — \}s*erépiïqu / t -T^J^^dt

[0041] With / A conjugate of a replica of the signal emitted by the emitting source A epliqueVJ (EM). - Calculation of the bistatic distance (d) and radial velocity (vr) of the target using the following formulas: dt^o and Vr JO

[0042] According to a preferred embodiment, the step of determining the angle of arrival 0O of the target proceeds to an estimation of the angle of arrival 0« of the target by means of a search for the maximum signal-to-noise ratio of the following function:

[0043] j~\ r iïEREtëREref*^

[0044] According to another alternative embodiment, the step of determining the angle of arrival of the target is an estimation of the angle of arrival ®t of the target by means of a search for the maximum signal-to-noise ratio of the following function: 100451

[0046] The focused signal Sr ( t, 0) then to be compressed in bistatic distance, Doppler according to a step of determining the bistatic distance and the radial velocity of the target.

[0047] Advantageously, the target detection and localization method further comprises a complementary step of optimizing the estimation of the target's angle of arrival 0O, said step consisting of carrying out the previous steps for another frequency channel. Brief description of the FIGURES

[0048] The invention will be better understood upon reading the following description, given solely by way of non-limiting example and made with reference to the accompanying drawings in which: - Fig. 1 is a schematic representation of a bistatic couple with one emitter and one receiver on which the invention is based. - Fig. 2 is a schematic representation of a receiver of the bistatic couple of Fig. 1, according to a non-limiting example of the invention. - Fig. 3 is a schematic comparative representation of the results obtained with a receiver according to the invention and receivers of the prior art. - Fig. 4 and Fig. 5 show comparative results between the localization carried out by means of a detection and localization device according to the invention and the actual localization provided by the GPS position extracted from cooperative drones.

[0049] It is understood that the embodiments described below are in no way limiting. In particular, variants of the invention may be conceived comprising only a selection of the features described below, isolated from the other features described, if this selection of features is sufficient to confer a technical advantage or to differentiate the invention from the prior art. This selection includes at least one preferably functional feature without structural details, or with only a portion of the structural details if this portion alone is sufficient to confer a technical advantage or to differentiate the invention from the prior art.

[0050] In particular, all the variants and all the embodiments described are combinable with each other if nothing prevents this combination from a technical point of view.

[0051] In the figures and in the rest of the description, elements common to several figures retain the same reference. Detailed description of the FIGURES

[0052] Figure 1 shows a bistatic couple consisting of an emitting source EM and a detection and localization device RE separated by a distance d(EM,RE), as well as a target C, distant from the emitting source EM by a distance d(EM,C), and distant from the device RE by a distance d(RE,C).

[0053] Passive detection in bistatic mode therefore involves the emitting source EM on the one hand and the detection and localization device RE on the other hand.

[0054] The emitting EM source is considered non-cooperative in that it emits signals whose destination and nature are not related to the use made of them by the RE device for detecting and locating targets C. These are, for example, FM band radio broadcasting signals intended for the broadcasting of public radio programs, or UHF or VHF broadcasting signals of the television type for example, or any other general-purpose broadcast.

[0055] A characteristic of these non-cooperative transmissions is that they are generally carried out within a defined frequency band, which is subdivided into disjoint sub-bands also called channels. For a given geographical region, each channel is allocated to a specific use, corresponding to a given type of transmission. For example, in the case of a transmitting source intended for broadcasting FM programs, the frequency band on which the transmitting source is likely to transmit extends from 87.5 MHz to 108 MHz and is subdivided into channels with a width of approximately 100 kHz. Within the band, all or part of the channels may be used.

[0056] Each channel used can also be intended for one or more uses such as the broadcasting of speech programs, music, images or multiplexed digital data.

[0057] Thus, the bandwidth of the FM signal transmitted on each channel is independent of that of the signals transmitted on the other channels and can vary over time depending on the signal or information being transmitted. For this reason, the EM transmitting source is described as non-cooperative and random.

[0058] The RE detection and localization device utilizes this EM emitting source by implementing a bistatic detection method that consists of separately receiving the direct signal emitted by the EM emitting source and receiving the waves backscattered by targets present in the area covered by the RE detection and localization device. Generally, for ease of implementation, this reception is limited to only one of the channels emitted by the EM emitting source. Indeed, the signals emitted on the different channels are not necessarily synchronous with each other, and their simultaneous processing requires appropriate processing.

[0059] The signal directly received from the emitting source EM is used as a reference by the detection and localization device RE to determine the time difference that the signal backscattered to the detection and localization device RE by the target C presents with respect to this reference. This difference allows, after processing, the determination of the bistatic distance of the target C, the velocity of the target C being determined elsewhere, in a conventional way, by Doppler processing.

[0060] Separate reception of the direct signal and the backscattered signal can be achieved by using a receiving element, such as a directional reference antenna, oriented towards the emitting source or not and another receiving element, such as an antenna, pointed in a given direction.

[0061] Separate reception of the direct signal and the backscattered signal can also be achieved by the same receiving element.

[0062] As represented in [Fig.2], the RE target detection and localization device C is configured to exploit in bistatic mode non-cooperative radio signals emitted on separate channels by an EM emitting source.

[0063] The RE device comprises a plurality N of ERE receiving elements, i.e. [1; N], as well as an ECU processing unit for the received radio signals.

[0064] The ERE receiving elements are substantially aligned along an axis (x) so as to define a broken line L, close to the main axis (x).

[0065] By "substantially aligned", we mean that the receiving elements ERE; form a linear network that is almost rectilinear in the sense that each receiving element does not deviate from the axis (x) by a distance greater than 25% of the length of the broken line L.

[0066] According to the invention, the receiving elements ERE; form a linear gap network insofar as the value of the spacing d(ERE;,EREi +1) between two successive receiving elements ERE;, EREi+i is greater than half the wavelength of said radio signals. This characteristic makes it possible to obtain good resolution.

[0067] According to the invention, the receiving elements ERE; form an irregular linear lacunar network insofar as V ie [1 ; N] and V ke [1 ; N], the value of the spacing d(EREi,EREk) between two distinct receiving elements ERE;, EREk is unique.

[0068] Thus, in this irregular network, the same spacing value is never found twice between two distinct receiving elements.

[0069] This feature makes it possible to minimize ambiguities on the angle of arrival of the target when the value of the spacing d(ERE;,EREi +[) between two successive receiving elements ERE;, EREi+i is greater than half the wavelength of said radio signals.

[0070] Advantageously, in order to minimize as much as possible the ambiguities on the angle of arrival of the target, the minimum difference between the values ​​of the spacings is at least equal to 10%.

[0071] Preferably, and for practical implementation reasons, the value of the spacing d(ERE;,EREi +[) between two successive receiving elements ERE;, EREi+i is shaped to exploit radio signals used for broadcasting analog and digital television or radio, after adaptation to transpose the dimensioning carried out in the television frequency band to the radio frequency band.

[0072] In order to allow the installation of the RE detection and localization device on the roof of a building, the value of the spacing d(EREi,EREN) between the first EREi and the last EREN receiving elements of the irregular gap linear network is preferably less than 20 m.

[0073] Advantageously, the target detection and localization device comprises at least 4 ERE receiver elements, at most 10 ERE receiver elements, preferably between 7 and 9 ERE receiver elements. This makes it possible to obtain an efficient device while minimizing the number of components, to ensure controlled cost and footprint with reasonable deployment complexity.

[0074] In order to cover all directions, i.e. an angular sector of 360°, several RE target detection and localization devices according to the invention can be installed, the whole then forming an SD target detection and localization system C.

[0075] In summary, this target detection and localization device, which constitutes a passive sector surveillance device, can be coupled with other similar devices to ensure omnidirectional protection, each device being capable of directly and precisely detecting and localizing targets of interest. The primary, but potentially expandable, application area relates to counter-drone operations using DVB-T / DVB-T2 transmitters.

[0076] DVB-T transmissions advantageously have a long integration time, typically between 0.5 seconds and 1 second.

[0077] The target detection and localization device employs a limited number of receiving elements (antennas), for example eight, and covers a sector, for example, of approximately + / -30°. Other limitations may also be considered.

[0078] The target detection and localization device is substantially linear and has a small footprint over a maximum distance of approximately ten meters. This limitation makes installation possible on any type of flat building terrace. This is compatible with a DVB-T frequency approximately between 500 MHz and 700 MHz and therefore with a wavelength of approximately 50 centimeters.

[0079] The target detection and localization device constitutes an irregular gap-filled antenna array.

[0080] The implementation of a target detection and localization device according to the invention will now be described.

[0081] The target detection and localization method C according to the invention uses a target detection and localization device exploiting non-cooperative radio signals emitted on separate channels by an EM emitting source, according to the invention.

[0082] The method comprises at least: - A 100-step reception process by each ERE receiver element of the target detection and localization device, of a signal originating from the source emitting EM along a direct path, and a signal backscattered by a target C in the covered space, - A selection step 200 allowing, for each ERE receiver element of the target detection and localization device, the separation and selection of at least one frequency channel to be used, - A step 300 to determine the bistatic distance d and the radial velocity vr of the target, - A step 400 for determining the angle of arrival of the target, based on measurements of the phase differences between the signals received by each receiving element ERE; of the target detection and localization device.

[0083] Preferably, and not limitingly, the step 300 for determining the bistatic distance d and the radial velocity vr of the target, is implemented by means of a distance-Doppler correlation operation on the received signal, with replicas of the signal emitted by the emitting source EM, affected by time and frequency shifts.

[0084] The implementation of step 300 makes it possible to determine, for a signal received at a given instant, the values ​​of the delay and the frequency shift v° for which a correlation peak is obtained between the measurement signal and the delayed reference signal, affected by a corresponding Doppler shift. A correlation peak is obtained when a target C located at a given distance from the detection and localization device RE and moving at a given speed, reflects back to the device RE the light emitted by the emitting source EM.

[0085] The time delay T° also allows the corresponding bistatic distance to be determined directly. This bistatic distance allows the target C to be located on an ellipsoid whose foci are the emitting source EM and the target detection and localization device. The determination of the target's Cartesian coordinates is itself obtained by knowing the angle θ of arrival of the target towards the target detection and localization device RE.

[0086]

[0034] The value f of the frequency shift for which a peak of correlation corresponds to the doppler frequency of the signal reflected by object 17 and is directly related to the doppler velocity Vd of object 17, relative to the detection system 12.

[0087] In particular, the step 300 of determining the bistatic distance d and the radial velocity vr of the target, implements the substeps described below.

[0088] First, the signal received by a receiver element ERE is expressed as a function of the signal received by a preselected reference receiver element EREref:

[0089]

[0090] With

[0091] sri; signal received by the receiving element ERE;,

[0092] signal emitted by the EM emitting source,

[0093] fo: frequency of the EM emitting source,

[0094] T°: bistatic delay (common to all ERE receiving elements;) between the path from the emitting EM source to the target, and the path from the target to the receiving ERE element;

[0095] dr0: derivative of the delay T" dt

[0096] _ £ d ■ Bistatic Doppler

[0097] d(EREi, EREref): value of the spacing between the receiving element EREi and the predetermined reference receiving element EREref

[0098] ©o: angle of arrival of the target common to all ERE receiving elements;

[0099] Indeed, analysis step 300 allows the distance parameters to be restored bistatic r, of bistatic Doppler v which is related to the derivative of the bistatic distance and angle of arrival with respect to the detection and localization device RE by proceeding as follows.

[0100] Within the received signal, the signal backscattered by the target can be approximated and schematized as follows on each of the receiving elements ERE; of the device RE:

[0101]

[0102] With

[0103] sri: signal received by the ERE receiving element;,

[0104] s«: signal emitted by the EM emitting source,

[0105] frequency of the EM emitting source,

[0106] l°: bistatic delay (common to all ERE receptor elements;) between the path from the emitting EM source to the target, and the path from the target to the receiving ERE element;

[0107] : derivative of the delay L° dt

[0108] _ j: Bistatic Doppler yo~ J° dt

[0109] : path difference between the path from target C to element receptor ERE; and the path from target C to reference receptor element EREref. The reference receptor element EREref is chosen arbitrarily.

[0110] As seen in FIGURE 2, = d(ERE; EREref) * cos(0j.

[0111] The expression for the received signal can therefore be rewritten as follows: [0H2]

[0113] Next, to estimate the parameters and % (common to all ERE receiving elements), it suffices to perform a so-called distance correlation operation, Doppler, according to the principle of finding the maximum power of the following function v) as a function of the hypotheses (t, v) as follows: [01141¢. (t, v) = Jrsy (t-Ti (O) s%repiit}Uit - T^J^dt

[0115] With M conjugate of a replica of the signal emitted by the emitting source eppliquex^J EM.

[0116] Since we are looking for a maximum in terms of energy ratio, the phase term does not need to be compensated.

[0117] Finally, the calculation of the bistatic distance d and the radial velocity vr of the target can be carried out using the following formulas:

[0118] d = CTo and =

[0119] The step of determining the angle of arrival 0" of the target consists of estimating the angle of arrival 00 of the target.

[0120] To do this, we perform a search operation to find the maximum signal-to-noise ratio of the following function:

[0121] i^f (KEREiEREref^Q) ^0) = LZ4^ v0)eJ^h------?------

[0122] The relevance of estimating the angle of arrival 0O of the target is intimately linked to the values ​​of the spacings between the receiving elements ERE;

[0123] Alternatively, the step 400 of determining the angle of arrival 0O of the target is an estimation of the angle of arrival 0O of the target by means of a search for the maximum signal-to-noise ratio of the following function:

[0124] Sr(t, ©) = E(t-Tf(t))>

[0125] the focused signal Sr ( t, 0 ) then to be compressed into bistatic distance, Doppler according to a determination step 300 of the bistatic distance d and the radial velocity vr of the target.

[0126] The target detection and localization method may further include an additional step 500 of optimizing the estimation of the angle of arrival 0() of the target.

[0127] This step may consist of repeating steps 100, 200, 300, 400 by choosing another frequency channel.

[0128] When the device is extended lengthwise while keeping the number of antennas unchanged, the antennas are spaced further apart, which leads to a greater risk of angular ambiguity, since overall the angular resolution is inversely proportional to the total extension of the device. This risk can then be content, by exploiting other frequency channels since the position of ambiguities varies with wavelength. The angular resolution is indeed inversely proportional to the total extent of the network since the phase shift term is:

[0129] „ d(EREi£REref)coi&) Joc

[0130] Fig. 3 establishes the contribution of irregular gap linear networks (dotted lines) according to the invention compared to classic linear networks (solid black line) and regular gap linear networks (solid grey line), in terms of isolation, resolution and ambiguity.

[0131] We recall that isolation represents the difference in levels between the true direction of a target and other hypotheses, resolution represents the ability to separate two "close" targets of the same level, and ambiguities represent other angle hypotheses with similar levels.

[0132] It appears that classical linear networks exhibit good isolation but average resolution, whereas regular linear gap networks exhibit improved resolution, but with ambiguities.

[0133] It can then be seen that the irregular gap linear networks according to the invention have an improved resolution, free from ambiguities.

[0134] Tests were carried out for a DVB-T frequency approximately between 500 MHz and 700 MHz with a wavelength of the order of 50 centimetres.

[0135] A conventional linear radar array would lead to spacing them by half the wavelength, resulting in a total extent of 7*0.25=1.75 meters. Furthermore, if, according to the conventional approach, an omnidirectional surveillance approach were considered, using a "circular" array with a target detection and localization device, this array extent would be reduced overall by a factor of two in terms of effectiveness for a given direction.

[0136] For a regular network of large extent, that is to say for an inter-antenna distance greater than half the wavelength, network lobes then appear which correspond to ambiguities on the direction of arrival of the targets.

[0137] With irregular gapless linear arrays according to the invention, with a maximum extension of the order of 8 to 9 meters, and a minimum spacing between two successive antennas of 60 centimeters, i.e. greater than lambda, only ambiguities of array lobes are observed outside the relatively constant elementary antenna lobe of the antennas.

[0138] The elementary antenna lobe is generally limited to -5 or -6 dB below the main peak level within the + / -60° main lobe, this lobe being twice the size of the monitoring sector. The monitoring sector is the area where detection is sought. targets, i.e. a lobe at -3dB. The extended lobe is the one where it is considered that contributions from other targets can still be received.

[0139] An antenna deployment then makes it possible to ensure these constraints over the entire DVB-T band, whose frequency channel bandwidth is 7.5 MHz, in order to facilitate implementation during deployments on various sites.

[0140] Irregular linear gap arrays according to the invention, with a maximum extension of approximately 8 to 9 meters, and a minimum spacing of 60 centimeters between two successive antennas, have been specified for a total extension of 8.28 meters, or 4.75 times the extension that would have been obtained with a conventional approach. The gain in accuracy obtained on the angular measurement is then close to this factor of 4.75 for the same system complexity.

[0141] A good resolution for the bistatic range (measured in DVB) of 40 meters was obtained, and the integration time (between 0.5 seconds and 1 second, typically for anti-drone operations) leads to a good resolution for the bistatic velocity (derived from the range) on the order of 1 meter / second. These performance levels on the range and velocity axes are generally accurate enough to contain only a single target under the assumption of bistatic range and its derivative.

[0142] A good estimation of the angle of arrival of the target with the resolution of the order of 4 degrees linked to the 8-meter extension network, was also obtained.

[0143] Thus, it is possible to separate two targets sharing the same assumption of bistatic distance and its derivative if they are sufficiently separated angularly.

[0144] We can therefore obtain detection plots with resolutions for the bistatic distance, the derivative of the bistatic distance, angle relative to the detection and target localization device, of the order of respectively 40 meters, Im / s, 4 degrees.

[0145] The accuracy is then deduced from these improved resolutions in inverse proportion to the square root of the signal-to-noise ratio of the targets.

[0146] The high accuracy of bistatic measurements, particularly of the bistatic distance and angle relative to the target detection and localization device (TUD), allows for the direct and precise determination, through a simple geometric transformation, of the target's Cartesian coordinates relative to the TUD, or any other reference point. The result of this direct transformation thus enables Cartesian localization, as opposed to tracking, which also uses velocity measurements to determine the target's Cartesian position and its kinematics. It should be noted that Cartesian tracking is also possible using measurements associated with a single bistatic pair, given their high resolution, particularly in terms of distance and angle.

[0147] FIGURES 4 and 5 show comparative results between the localization carried out by means of a detection and localization device according to the invention and the actual localization provided by the GPS position extracted from cooperative drones.

[0148] Figures 4 and 5 illustrate, respectively, the angle relative to the antenna array and the Cartesian distance relative to the receiver. The measurements obtained are compared to estimates derived from a GPS trajectory reference of a drone.

[0149] Among the main advantages of this invention, it is noted that the target detection and localization devices according to the invention reduce the impact of the blind sector related to the axis of the emitter.

[0150] These devices also allow direct bistatic localization in the device's frame of reference with good horizontal accuracy, on the order of 30 m along the two axes of the horizontal plane.

[0151] Furthermore, the implementation of the devices according to the invention allows for instant localization as soon as a detection takes place, which makes it possible to launch an alert instantly since it does not require simultaneous detections on other transmitter / receiver pairs.

[0152] The installation and implementation of monitoring using this principle is simpler than systems based on multistatic association, since it is possible to centralize the detection system.

[0153] Localization is directly accessible from a single transmitter / receiver pair, which simplifies the deployment of the devices according to the invention compared to a conventional passive solution that requires the combination of three bistatic pairs. Indeed, the density of DVB transmitters does not necessarily allow the use of three separate transmitters for certain areas of interest.

[0154] It is also possible to combine several devices according to the invention for omnidirectional coverage needs while preserving the independence of the performance of each sector.

[0155] The detection and localization method and device according to the invention allow for a simplification and improvement of the performance of the principle of associating bistatic markers with a physical object when dealing with a Single Frequency Network. Indeed, the results can be based on assumptions of track proximity rather than on assumptions of marker association.

[0156] The level of bistatic localization accuracy obtained with the detection and localization method and device according to the invention exhibits a measurement precision sensitive to wind direction, which can allow a certain level of classification.

[0157] The detection and localization method and device according to the invention is adaptable to a 3D extension, by adding the vertical component.

[0158] Tracking performed from simple bistatic information makes it possible to overcome possible imperfections when associating poorly georeferenced data.

[0159] Given the accuracy of the measurement, the kinematic parameters of the targets are particularly well estimated, which helps to sort the plots between the desired plots and the others even if some false alarms may remain.

[0160] Similarly, the increased precision of the measurement makes it possible to: - Limit the elimination of roadside detections to a smaller sector, - Limit false alarms related to the selection of hypotheses (distance, angle) with a drone-like signature,

[0161] Of course, the invention is not limited to the examples just described.

Claims

Demands

1. A target detection and localization (TR) device (C) configured to exploit in bistatic mode non-cooperative radio signals emitted on separate frequency channels by a transmitting source (TS), the device comprising a plurality N of at least two, preferably at least three, receiving elements (ERE; ), i.e. [1; N], as well as a processing unit (ECU) of said radio signals, characterized in that: - The receiving elements (ERE;) are substantially aligned along an axis (x) and, - The value of the spacing d(ERE;,EREi +[) between two successive receiving elements (ERE;, EREi+i) is greater than half the wavelength of said radio signals, and, - V ie [1; N] and V ke [1; N], the value of the spacing d(EREi,EREk) between two distinct receiving elements (ERE;, EREk) is unique.

2. Target detection and localization (RE) device (C) according to claim 1, characterized in that the minimum difference between the spacing values ​​is at least equal to 10%.

3. Target detection and localization (RE) device (C) according to claim 1 or 2, characterized in that the value of the spacing d(EREi,EREi +[) between two successive receiving elements (ERE;, EREi+i) is shaped to exploit radio signals used for broadcasting analog and digital television or radio.

4. Target detection and localization (RE) device (C) according to any one of the preceding claims, characterized in that the value of the spacing d(EREb EREN) between the first (EREi) and last (EREN) receiving elements is less than 20 m.

5. Target detection and localization (TR) device (C) according to any one of the preceding claims, characterized in that it comprises at least 4 receiver elements (REE;), at most 10 receiver elements (REE;), preferably between 7 and 9 receiver elements (ERE;).

6. Target detection and localization (SD) system (C) comprising a plurality of target detection and localization (RE) devices conforming to any one of claims 1 to 5 and arranged to cover an angular sector of 360°.

7. A method for detecting and locating targets (C) by means of a target detection and locating device (RE) exploiting non-cooperative radio signals emitted on separate frequency channels by a transmitting source (EM), said device conforming to any one of claims 1 to 6, said method comprising at least: - A reception step (100) by each receiving element (ERE) of the target detection and locating device, of a signal from the transmitting source (EM) along a direct path, and of a signal backscattered by a target (C) in the covered space, - A selection step (200) enabling, for each receiving element (ERE;) of the device (RE), the separation and selection of at least one frequency channel to be exploited, - A step of determining the bistatic distance (d) and the radial velocity (vr) of the target, itself derived from said bistatic distance (d), - A step of determining the angle of arrival 0O of the target, from the measurements of the phase differences between the signals received by each receiving element (ERE; ) of the device (RE).;

8. Method for detecting and locating targets (C) according to claim 7, characterized in that the step of determining the bistatic distance (d) and the radial velocity (vr) of the target, is implemented by means of a distance-Doppler correlation operation on the received signal, with replicas of the signal emitted by the emitting source (EM), affected by time and frequency shifts.

9. A method for detecting and locating targets (C) according to claim 8, characterized in that the step of determining the bistatic distance (d) and the radial velocity (vr) of the target, implements the following substeps:

10. - The expression of the signal received by a receiving element (ERE; ) as a function of the signal received by a preselected reference receiving element (EREref): With sri; signal received by the receiving element (ERE;), se; signal emitted by the emitting source (EM), fo: frequency of the emitting source (EM), r°: bistatic delay (common to all receiving elements (ERE;)) between the path from the emitting source (EM) to the target, and the path from the target to the receiving element (ERE;), : derivative of the delay r° dl _ : Bistatic Doppler VO — JV dt d(EREi, EREref): value of the spacing between the receiving element (EREi) and the predetermined reference receiving element (EREref) 0O: common target arrival angle for all receiving elements (ERE;) - Estimation of the parameters r» and by means of the search for the maximum power of the function ¢,(7 v); With (A conjugate of a replica of the signal emitted by the emitting source (EM). - Calculation of the bistatic distance (d) and radial velocity (vr) of the target using the following formulas: d = cfo and Vr = y^o Method for detecting and locating targets (C) according to claim 9, characterized in that the step of determining the angle of arrival 0O of the target, performs an estimation of the angle of arrival 0O of the target by means of a search for the maximum signal-to-noise ratio of the following function: / \ V < n-rf d(EREi£REref)cosi&) M\0 / = t[To. »-------c— ----

11. Method for detecting and locating targets (C) according to claim 9, characterized in that the step of determining the angle of arrival 0O of the target, is an estimation of the angle of arrival 0O of the target by means of a search for the maximum signal-to-noise ratio of the following function: c (. fd(EREi^REref)coi&) S^t, 0) - ( t-Tf ( t ) ) The focused signal Sr ( t, 0 ) is then to be compressed into bistatic, Doppler distance according to a step of determining the bistatic distance (d) and the radial velocity (vr) of the target.

12. Method for detecting and locating targets (C) according to claim 10 or 11, characterized in that it further comprises a complementary step (500) for optimizing the determination of the arrival angle 0O of the target, said step consisting of carrying out the steps of claims 9 and 10 or 9 and 11 for another frequency channel.

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